diff --git a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
--- a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
+++ b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
@@ -270,25 +270,42 @@
     assert(Buffer->IsComplete && "serialization did not complete");
     auto &SerializedAST = Buffer->Data;
     auto Size = SerializedAST.size();
-    auto Int8Ty = llvm::Type::getInt8Ty(*VMContext);
-    auto *Ty = llvm::ArrayType::get(Int8Ty, Size);
-    auto *Data = llvm::ConstantDataArray::getString(
-        *VMContext, StringRef(SerializedAST.data(), Size),
-        /*AddNull=*/false);
-    auto *ASTSym = new llvm::GlobalVariable(
-        *M, Ty, /*constant*/ true, llvm::GlobalVariable::InternalLinkage, Data,
-        "__clang_ast");
-    // The on-disk hashtable needs to be aligned.
-    ASTSym->setAlignment(llvm::Align(8));
-
-    // Mach-O also needs a segment name.
-    if (Triple.isOSBinFormatMachO())
-      ASTSym->setSection("__CLANG,__clangast");
-    // COFF has an eight character length limit.
-    else if (Triple.isOSBinFormatCOFF())
-      ASTSym->setSection("clangast");
-    else
-      ASTSym->setSection("__clangast");
+
+    if (Triple.isOSBinFormatWasm()) {
+      // Emit __clangast in custom section instead of named data segment
+      // to find it while iterating sections.
+      // This could be avoided if all data segements (the wasm sense) were
+      // represented as their own sections (in the llvm sense).
+      // TODO: https://github.com/WebAssembly/tool-conventions/issues/138
+      llvm::NamedMDNode *MD =
+          M->getOrInsertNamedMetadata("wasm.custom_sections");
+      llvm::Metadata *Ops[2] = {
+          llvm::MDString::get(*VMContext, "__clangast"),
+          llvm::MDString::get(*VMContext,
+                              StringRef(SerializedAST.data(), Size))};
+      auto *NameAndContent = llvm::MDTuple::get(*VMContext, Ops);
+      MD->addOperand(NameAndContent);
+    } else {
+      auto Int8Ty = llvm::Type::getInt8Ty(*VMContext);
+      auto *Ty = llvm::ArrayType::get(Int8Ty, Size);
+      auto *Data = llvm::ConstantDataArray::getString(
+          *VMContext, StringRef(SerializedAST.data(), Size),
+          /*AddNull=*/false);
+      auto *ASTSym = new llvm::GlobalVariable(
+          *M, Ty, /*constant*/ true, llvm::GlobalVariable::InternalLinkage,
+          Data, "__clang_ast");
+      // The on-disk hashtable needs to be aligned.
+      ASTSym->setAlignment(llvm::Align(8));
+
+      // Mach-O also needs a segment name.
+      if (Triple.isOSBinFormatMachO())
+        ASTSym->setSection("__CLANG,__clangast");
+      // COFF has an eight character length limit.
+      else if (Triple.isOSBinFormatCOFF())
+        ASTSym->setSection("clangast");
+      else
+        ASTSym->setSection("__clangast");
+    }
 
     LLVM_DEBUG({
       // Print the IR for the PCH container to the debug output.
diff --git a/clang/test/PCH/pch-wasm.c b/clang/test/PCH/pch-wasm.c
new file mode 100644
--- /dev/null
+++ b/clang/test/PCH/pch-wasm.c
@@ -0,0 +1,7 @@
+// REQUIRES: webassembly-registered-target
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown-wasm -emit-pch -fmodule-format=obj %S/pchpch1.h -o - | llvm-objdump --section-headers - | FileCheck %s
+
+// Ensure that clangast section should be emitted in a section for wasm object file
+
+// CHECK: file format wasm
+// CHECK: __clangast   {{[0-9a-f]+}} {{[0-9a-f]+}}
diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -370,11 +370,33 @@
   // The position where the section header ends, for measuring its size.
   Section.PayloadOffset = W->OS.tell();
 
-  // Custom sections in wasm also have a string identifier.
-  writeString(Name);
+  // Custom sections in wasm also have a string identifier with extra paddings
+  // for alignment for special sections.
+  // TODO: support section alignment at asm and llvm level?
+  unsigned Alignment = 1;
+
+  // The on-disk hashtable in clangast needs to be aligned by 4 bytes.
+  if (Name == "__clangast")
+    Alignment = 4;
+
+  // Calculate the encoded size of name length and add pads based on it and
+  // alignment.
+  raw_null_ostream NullOS;
+  uint64_t NameSizeLength = encodeULEB128(Name.size(), NullOS);
+  uint64_t ContentsOffset =
+      Section.PayloadOffset + NameSizeLength + Name.size();
+  uint64_t Paddings = offsetToAlignment(ContentsOffset, Align(Alignment));
+  ContentsOffset += Paddings;
+
+  // LEB128 greater than 5 bytes is invalid
+  assert((NameSizeLength + Paddings) <= 5 && "too long section name to align");
+
+  encodeSLEB128(Name.size(), W->OS, NameSizeLength + Paddings);
+  W->OS << Name;
 
   // The position where the custom section starts.
   Section.ContentsOffset = W->OS.tell();
+  assert(Section.ContentsOffset == ContentsOffset && "invalid padding");
 }
 
 // Now that the section is complete and we know how big it is, patch up the
diff --git a/llvm/test/MC/WebAssembly/custom-section-alignment.ll b/llvm/test/MC/WebAssembly/custom-section-alignment.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/custom-section-alignment.ll
@@ -0,0 +1,10 @@
+; RUN: llc -filetype=obj %s -o - | od -t x1 -v | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+!0 = !{ !"before", !"\de\ad\be\ef" }
+!1 = !{ !"__clangast", !"\fe\ed\fa\ce" }
+!wasm.custom_sections = !{ !0, !1 }
+
+; Ensure that __clangast content is aligned by 4 bytes
+; CHECK: {{(([0-9a-f]{2} ){4})*}}fe ed fa ce